Rescaling for Compression

Sometimes when coring, cores can become compressed in the chamber. For example, if you know you’ve started a drive at 1 m, and finished at 2 m, but only collected 0.95 m of sediment, you may have compression. This is often a problem for Livingstone-type corers, but not an issue with Russian-types. This can be caused by:

  • stretch in locking lines for corer heads (or ground compression if those lines are locked to the surface),
  • friction of sediment inside the tube,
  • airlocks inside the tube,
  • poor cutting performance of the corer end.

It’s difficult to know whether you’ve lost material or suffered compression, so good practice says the stratigraphy should be cross-correlated with an adjacent sequence. If there is compression, this can be corrected for, but in practice it can be a pain. Here’s a simple way of doing it in R. Here I’m correcting the depths for sub-samples taken at 10 mm intervals, in a core that should have been 1 m in length, from 1 m to 2 m depth, but was actually 0.95 m in length when extracted.

length <- 950     # the measured length of the core when extracted
truestart <- 1000 # the starting depth of the drive
trueend <- 2000   # the finishing depth of the drive
intervals <- 10   # the sampling interval

# create the sampling sequence - this may already be available in your data
z <- seq(from = trueend-length, to = trueend, by = intervals)

# correct those depths
zc <- seq(from = truestart, to = trueend, length.out = length(z))

# compare the data
df <- data.frame(original = z,
                 corrected = round(zc, digits = 0),
                 difference = round(z-zc, digits = 0)

This code assumes that there’s more compression at the top, and less at the bottom of the sequence, which is not unreasonable given the possibles causes listed above. Sometimes cores can expand due to decompression of the sediment following coring. This is particularly common in gytjja from lakes, where the weight of the water column compresses the sediment in-situ, thus it expands ex-situ. Take an example where we have a core of length 1.05 m, but we know we only cored from 1 m to 2 m; we need to compress the core back to 1 m length. The following code deals with this problem:

length <- 1050    # the measured length of the core when extracted
truestart <- 1000 # the starting depth of the drive
trueend <- 2000   # the finishing depth of the drive
intervals <- 10   # the sampling interval

# create the sampling sequence - this may already be available in your data
z <- seq(from = truestart, to = truestart+length, by = intervals)

# shift the depths so they are centered
zs <- z-(max(z)-trueend)/2

# correct these depths
zc <- (trueend-truestart)/length * zs + mean(c(trueend, truestart)) - ((trueend-truestart)/length) * mean(zs)

# compare the data
df <- data.frame(original = z,
                 shifted = zs,
                 corrected = round(zc, digits = 0),
                 difference = round(zs-zc, digits = 0)

Official New Divisions for the Holocene

It’s official! The I.C.S. have announced the subdivisions for the Holocene, so I’ve made a helpful graphic to summarise their announcement, which I’ve reproduced in full below.

A high resolution copy can be found on the resources page.

Formal subdivision of the Holocene Series/Epoch

It has been announced that the proposals for the subdivision of the Holocene Series/Epoch (11 700 years ago to the present day) into three stages/ages and their corresponding subseries/subepochs by the International Subcommission on Quaternary Stratigraphy (ISQS) (a subcommission of the International Commission on Stratigraphy – ICS) have been ratified unanimously by the International Union of Geological Sciences (IUGS). The subdivisions now formally defined are:

1. Greenlandian Stage/Age = Lower/Early Holocene Subseries/Subepoch
Boundary Stratotype (GSSP): NorthGRIP2 ice core, Greenland (coincident with the Holocene Series/Epoch GSSP, ratified 2008). Age: 11,700 yr b2k (before AD 2000).

2. Northgrippian Stage/Age = Middle/Mid-Holocene Subseries/Subepoch
Boundary Stratotype (GSSP): NorthGRIP1 ice core, Greenland. Global Auxiliary Stratotype: Gruta do Padre Cave speleothem, Brazil. Age: 8326 yr b2k.

3. Meghalayan Stage/Age = Upper/Late Holocene Subseries/Subepoch
Boundary stratotype (GSSP): Mawmluh Cave speleothem, Meghalaya, India. Global Auxiliary Stratotype, Mount Logan ice core, Canada. Age: 4250 yr b2k.

These divisions are now each defined by Global Stratotype Sections and Points (GSSPs), which means that they are fixed in time in sedimentary sequences. The terms Greenlandian Stage/Age, Northgrippian Stage/Age, Meghalayan Stage/Age, Lower/Early Holocene Subseries/Subepoch, Middle/Mid-Holocene Subseries/Subepoch and Late/Upper Holocene Subseries/Subepoch therefore have formal definitions and boundaries.

These definitions represent the first formal geological subdivision of the Holocene Series/Epoch, resulting from over a decade of labour by members of the joint ISQS (International Subcommission on Quaternary Stratigraphy) – INTIMATE Members Working Group (Integration of Ice-core, Marine and Terrestrial Records), led by Professor Mike Walker (University of Aberystwyth).

Phil Gibbard
Secretary General ICS
Cambridge 26.6.18

Fake Morgan Dollar & Pound Coin – Metallurgy

The Morgan silver dollar is a precious metal coin minted in the late 19th century into the early 20th century. They are collectable and thus occasionally counterfeited. I’ve recently analysed genuine and counterfeit silver dollars using x-ray fluorescence (XRF) – here I’ve used a Rigaku NEX-CG ED-XRF instrument using a “fundamental parameters” estimation mode. There’s some work to do to optimise the system for these samples – it’s not something we usually work on!

Anyhow, the counterfeit seems to be brass, plated with silver. The real thing has a specification of 90% Ag, 10% Cu.

 Element Specification Genuine  Counterfeit
Ag 90 % 92.3 % 27.2 %
Cu 10 % 6.6 % 43.1 %
Zn 0 % 0.0 % 26.0 %

I’ve also analysed some old “round-pound” £1 coins. These were demonetised in 2017, because nationally around 1 in 30 of the coins in circulation were counterfeit – I’ve heard that in some areas, the number could have been as high as 1 in 5. I’ve found that genuine and counterfeit coins have a pretty similar Nickel-Brass metal, but can be easily distinguished by their Pb content – around 1.5 % in counterfeits, and negligible in genuine coins. The Pb is added to improve malleability.

Specification Genuine (1 σ, n = 8) Counterfeit (1 σ, n = 2) 
Ni 5.5 % 6.0 ± 0.5 % 4.9 ± 0.1 %
Cu 70.0 % 69.9 ± 0.6 % 67.3 ± 0.4 %
Zn 24.5 % 23.1 ± 0.9 % 25.1 ± 0.1 %
Pb  0.0 % 0.0 ± 0.0 % 1.4 ± 0.6 %

I’m going to develop this into part of an outreach activity that uses our Niton Handheld XRF. I’m planning demonstrations of contaminated vs. uncontaminated soils, and some other geological samples.

Illustrating the Mutual Climate Range (MCR) Method

I’m writing up some lecture material on transfer functions and environmental reconstruction, and needed to draw a diagram to illustrate the mutual climatic range method. This code could also be used to make simple MCR calculations and the like.

An example of the output. Because the code produces random patterns, it doesn’t always produce overlapping ranges. It can simply be run a few times until the illustration looks good!

Read more

Tyre Depth Gauge for Loose Powder XRF Preps

When we prepare samples for analysis by XRF we tend to use a simple pressed powder preparation method in the first instance. The depth of the sample is important for  calculating matrix density and in correcting for non-infinite depth samples (thin samples and/or those with a low average atomic number). We used to use a standard digital caliper, but it wasn’t ideal. The 150 mm calipers we had were unwieldy, and it care was needed to accurately level the depth gauge probe.

Erroneously small sample heights could be recorded using the depth gauge on a standard caliper (left). Using a digital tyre gauge (right) results in a more accurate measurement.


150 mm digital calipers (top) and a digital tyre depth gauge (bottom).

Enter the digital tyre depth gauge! These digital tyre depth gauges are easy to find online for a few quid. They have a nice wide guide, and as you can see with a range of up to 25 mm, they are perfect for measuring the depth in XRF loose-powder pots (we use pots that are nominally 22 mm tall). The method goes:

  1. Zero the display whilst measuring the depth of the empty pot.

    Measure from the top of the pot to the base, and zero the display.
  2. Fill the pot with sample, press and weigh.
  3. Measure to the top of the sample from the top of the pot.

    Having filled the pot with the sample, probe from the top of the pot to the sample surface.
  4. The height of the sample is the additive inverse of the reading (that is, -7.45 mm is 7.45 mm).

How The Itrax Core Scanner Works – New Poster

Having described the basic mode of operation of the Itrax core-scanner countless times in the past few months, I thought it time for a proper poster with nice diagrams, so I recently made this poster for the Itrax facility I’m looking after. It has a description of how x-ray beams are created and controlled in the scanner, the principles of measurement, and an explanation of how deriving chemical compositional data from fluorescence spectra can be done. It’s available from the resources page of this website. Let me know if you print one and use it – I love seeing photos of resources I’ve created being used!

If you need more information on the Itrax core scanner, there’s this paper and this edited volume that may be of interest.

The poster, printed and hung-up in the laboratory


I took a trip with our second-year students enrolled on Phil Hughes’ GEOG20351 “Glaciers” course to the Arenigs in North Wales. I delivered a short presentation on work I did as an undergraduate in 2008 on a nearby lacustrine sequence with an excellent chironomid temperature proxy record that seems to date to the late-glacial and early Holocene. I’ve uploaded the document here if anyone wants to take a closer look.

Cedar Pollen Size Paper

Ben Bell, a PhD candidate has just published his latest collaborative research on cedar pollen and climate variability. His research is focussed on ways in which Cedrus atlantica might be used as a proxy for (palaeo)climate in the Atlas. This paper examines a previously postulated link between pollen grain size and moisture availability, and concludes that moisture availability is not a significantly related to grain size in this context.

The study makes use of a number of methods for determining grain size – light microscopy, scanning electron microscopy, and laser granulometry. I was involved in the laser granulometry aspect, which Ben proves experimentally is comparable to the microscopic methods. Laser granulometry is considerably less time consuming than microscopic examination, so allowed for the large sample size used in this study.

The paper is published in Palynology, and is open access, available here.